With respect to relatively small scale iron-manufacturing of a wide variety of products in small quantities, a method has been developed for producing granulated metallic iron by agglomerating a material mixture including an iron-oxide-containing material (iron source) such as iron ore and a carbonaceous reducing agent such as coal, heating the agglomerated material mixture in a moving hearth-type reducing furnace for solid reduction, and cooling produced hot granulated metallic iron while separating them from slag generated as a by-product. The hot granulated metallic iron is cooled in a cooler to where the hot granulated metallic iron is transferred by a feeder from the moving hearth-type reducing furnace. The inside of the cooler is indirectly cooled by a flow of water over the exterior surface. The hot granulated metallic iron fed into the cooler is cooled while its relative position is changed during its passage through the inside of the cooler, and then is discharged from the cooler as granulated metallic iron.
The temperature of the hot granulated metallic iron at the time it is fed into the cooler is about 900 to 1000° C. The hot granulated metallic iron is cooled to about 150° C. in the cooler and then is discharged from the cooler. In the case that the temperature of the granulated metallic iron when it is discharged from the cooler is higher than 150° C., red rust tends to be generated on the surface of the granulated metallic iron by the reaction of moisture in the air with the granulated metallic iron. Therefore, in order to adequately cool the hot granulated metallic iron in the cooler, the total length of the cooler must be enlarged or the time the hot granulated metallic iron takes to pass through the cooler must be extended by decreasing the passing speed of the hot granulated metallic iron. However, facility development is necessary for the enlargement of the total length of the cooler and as a consequence, the facility scale is expanded. Thus, space cannot be saved. Furthermore, the decrease in the passing speed of the hot granulated metallic iron in the cooler decreases the productivity. Additionally, the increase in the temperature of the inside of the cooler might be prevented by increasing the water amount flowing over the exterior surface of the cooler, but the decrease in the temperature achieved by increasing the water amount is negligible.
Meanwhile, the resulting granulated metallic iron after the cooling may be left outdoors due to the imbalance in supply and demand. When the granulated metallic iron is left to stand for a long period of time, red rust may occur on the surface of the granulated metallic iron. The occurrence of red rust degrades the appearance of the granulated metallic iron thus decreasing the commercial value. Furthermore, the iron source is consumed with the occurrence of red rust; which leads to loss of the iron source. Thus, granulated metallic iron which is highly resistant to red-rusting has been desired.
Japanese Unexamined Patent Application Publication No. 3-268842 previously filed by the present applicants does not relate to a technology for preventing the occurrence of red rust in granulated metallic iron produced by a moving hearth-type reducing furnace, but provides a method for producing pig iron for casting. This patent application discloses that the occurrence of red rust can be prevented by forming a coating of magnetite on the surface of the pig iron by cooling foundry pig iron using mist or water vapor. However, the pig iron demolded from a casting mold is piled up on a carriage, and mist or water vapor is applied to the pig iron in this condition. Therefore, in this technology, the entire surface of the iron pig cannot be prevented from red-rusting.